In many cases, the value of a device for controlling a semiconductor device in accordance with an instruction signal input from the outside to supply current to a motor and drive the motor is enhanced by providing the device with such a protecting function that overcurrent (current extremely larger than stationary current) flowing when the motor is set to a lock state is detected and an outputting operation of the motor is restricted or stopped. In the driving device thus constructed, there is a case where when the voltage applied to the motor is drastically varied by the switching of the instruction signal supplied from the outside, the variation of the rotational number of the motor (=the electromotive force of the motor) cannot follow the variation of the applied voltage and inrush current flows transiently. Accordingly, it is supposed that the inrush current is erroneously judged as the above overcurrent.
As a technique of preventing the erroneous judgment of the overcurrent as described above is known a technique of reducing the inrush current by delaying the response of the motor applied voltage to the variation of an input signal, that is, gradually increasing the motor applied voltage (so-called slow start system). This system is effective to prevent the erroneous judgment of the protection function of the overcurrent, however, it is difficult to apply it to a system to which relatively high response is required. For example, supposing that the above system is applied to a system for driving a blower motor of an air conditioner mounted in a vehicle, a delay time occurs from the time point at which a user carries out an operation of changing an air blowing amount till the air blowing amount of the air conditioner is actually varied, and thus the commodity performance is lowered.
For example, JP-A-2001-45790 discloses a technique of allowing the inclination of the slow start to be set by two stages and switching the response so that the response is lowered for a fixed time from the start time of the motor and then the response is increased.
However, in the technique disclosed in the above publication, it is merely supposed that the switching operation of the response is carried out only at the start time of the motor, and the subsequent switching operation of the input signal is carried out with high response at all times. Accordingly, the erroneous judgment problem of the overcurrent protection function to the inrush current as described above can be never solved.
Furthermore, the easiness of the flow of the inrush current is also varied by the rating or type of the motor, the load state of the motor, the ambient temperature of the motor or the like. However, in the technique disclosed in the above publication, the response is lowered at the start of the motor at all times, and thus the start of the motor is delayed at all times irrespective of whether the inrush current flowing at the start of the motor actually occurs at such level that the erroneous judgment occurs.